1. Field of the Invention
The present invention relates to a tensioner with a ratchet mechanism having a circlip member that engages with the rack teeth formed on the outer circumferential surface of a piston. More particularly, the invention relates to an improvement in structure to decrease an impact sound that occurs when the external force from the chain causes the piston to retract.
2. Description of Related Art
Generally, in timing chains for valve drives of internal combustion engines, camshaft chains in use for a camshaft-camshaft drive, and balancer chains, have tensioners that are used on the slack side of a chain to take up slack in the chain and to apply tension to the chain.
During operation, a piston of the tensioner presses against the chain to maintain tension in the chain. Also, during operation, when tension in the chain increases due to resonance of a chain span, an excessive shock load from the chain acts on the piston of the tensioner, causing the piston to retract into the housing of the tensioner.
In order to prevent such a retraction of the piston, a tensioner device with a ratchet mechanism has been used as shown in Japanese patent application laid-open publication No. 2004-353758.
The ratchet mechanism includes rack teeth formed on the outer circumferential surface of the piston, a pawl member housed in a pawl hole that extends perpendicular to a piston bore having pawl teeth engageable with the rack teeth of the piston, and a pawl coil spring that biases the pawl member into engagement with the rack teeth of the piston.
During operation, when the piston moves in the protruding direction, the rack teeth of the piston travel over the teeth of the pawl member and permit the movement of the piston in the protruding direction. When the piston moves in the retracting direction, the rack teeth of the piston engage with the pawl teeth of the pawl member, and the pawl member comes into contact with an end surface of the pawl hole with the rack teeth of the piston engaged with the pawl teeth of the pawl member, thereby restricting the movement of the piston in the retracting direction. When the prior art piston retracts, the pawl member traveling along with the piston impacts the end surface of the pawl hole and generates an impact sound.
Therefore, there is a need for a tensioner that is directed to decreasing the impact sound generated at the time of retraction of the piston of the tensioner with a ratchet mechanism.